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6. P. Rodriguez and M. T. M. Koper, Phys Chem Chem Phys, 2014, 16, 13583-13594. 460. 7. S. Cherevko, A. R. Zeradjanin, G. P. Keeley and K. J. J. Mayrhofer, ...

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This article can be cited before page numbers have been issued, to do this please use: E. Pizzutilo, S. Freakley, S. Geiger, C. baldizzone, A. Mingers, G. Hutchings, K. J. J. Mayrhofer and S. Cherevko, Catal. Sci. Technol., 2017, DOI: 10.1039/C7CY00291B. Volume 6 Number 1 7 January 2016 Pages 1–308

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Addressing stability challenges of using bimetallic

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electrocatalysts: the case of gold-palladium nanoalloys

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Enrico Pizzutiloa*, Simon J. Freakleyb, Simon Geigera, Claudio Baldizzoneb, Andrea

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Mingersa, Graham J. Hutchingsb, Karl J. J. Mayrhofera,c,d, Serhiy Cherevkoa,c*

6 aDepartment

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of Interface Chemistry and Surface Engineering, Max-Planck-Institut für

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Eisenforschung GmbH,

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Max-Planck-Strasse 1, 40237 Düsseldorf, Germany

10 bCardiff

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Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT

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Forschungszentrum Jülich GmbH, Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (IEK-11), Egerlandstr. 3, 91058 Erlangen, Germany

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Department of Chemical and Biological Engineering, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany

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*Corresponding

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Tel.: +49 211 6792 160, FAX: +49 211 6792 218

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authors: [email protected] [email protected]



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DOI: 10.1039/C7CY00291B

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Abstract

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Bimetallic catalysts are known to often provide enhanced activity compared to the pure metals, due to electronic, geometric and ensemble effects. However, applied catalytic reaction conditions may induce re-structuring, metal diffusion and dealloying. This gives rise to a drastic change in surface composition, thus limiting application of bimetallic catalysts in real systems. Here we report a study on dealloying using an AuPd bimetallic nanocatalyst (1:1 molar ratio) as a model system. The changes in surface composition over time are monitored in-situ by cyclic voltammetry, dissolution is studied in parallel with an online inductively coupled plasma mass spectrometry (ICP-MS). It is demonstrated how experimental conditions such as different acidic media (0.1 M HClO4 and H2SO4), different gases (Ar and O2), upper potential limit and scan rate significantly affect the partial dissolution rates and consequently the surface composition. The understanding of these alterations is crucial for the determination of fundamental catalyst activity, and plays an essential role for real applications, where long term stability is a key parameter. In particular, the findings can be utilized for the development of catalysts with enhanced activity and/or selectivity.

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Keywords: bimetallic, dealloying, dissolution, catalysis, Au, Pd

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1 Introduction

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Since discovery, gold catalysis has received ever-increasing interest and recently was proven to be a very successful tool for organic synthesis 1-5. In electrochemistry, gold has been commonly used as electrode for various applications and fundamental studies, thanks to its chemical inertness in the stability potential window of water and resistance to oxide formation. The “revival” of gold has attracted the attention in the electrocatalysis community, as it reveals interesting activity for carbon monoxide oxidation, alcohol oxidation and oxygen reduction reaction 6. In most of the applications, gold is typically considered to be completely stable, however dissolution cannot always be neglected 7.

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Recent work in heterogeneous catalysis showed that the addition of palladium to gold leads to activity or selectivity enhancement in several reactions 1, 8-12 (alcohol oxidation, H2O2 synthesis, toluene oxidation to benzoyl benzoate, methane oxidation with H2O2), and led to new exciting catalyst development based on bimetallic nanoparticle catalysts. Over the last years, gold palladium alloys have also been commonly studied in electrocatalysis, i.e. as catalysts for the oxygen reduction reaction 13-16, hydrogen peroxide synthesis and reduction 17-19, ethanol oxidation 20-22 and methanol oxidation 23. 2

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In homogeneous catalysis the transmetalation from Gold to Palladium in order to combine the typical gold-catalyzed cycloisomerization with additional C-C cross coupling, 8, 24-26 and this also has been studied computationally in great detail. 27

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It is known that bimetallic catalysts often provide enhanced activity compared to their pure counterparts and many recent studies have focused particularly on this topic. The superior activity can result from three different effects. (I) The electronic or ligand effect which causes changes in the band structure, thus influencing the strength of the binding between the metal surface and adsorbate molecules 28-30. (II) The geometric effect produces surface strain as a consequence of atomic arrangement of surface atoms to reduce the lattice mismatch 31, 32. (III) An ensemble effect arises when individual or small groups (ensembles) of different metal atoms on the surface act as preferential active sites available to adsorbates 33, 34. Indeed, the co-presence of both metals impacts the reaction rates and kinetics. Among other, this is particularly relevant for applications such as hydrogen peroxide synthesis (AuPd 1, 17, 18, 35, PtHg 36, PdHg 37), CO oxidation (AuPd 38), ethanol oxidation (AuPd 39), methanol oxidation (Pt-M 40) and formic acid oxidation (Pt-M 41, 42).

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The surface compositions along with the surface atomic arrangement in bimetallic nanoparticles play an extremely important role and are often crucial for high reactivity 17, 22, 43, 44. However, despite the great excitement around these catalysts, it is a great challenge to control activity over extended times only by tuning composition and structure during synthesis. Indeed, the reaction environment and the applied conditions 45-47 play a key role in the stability and thus the success and future application of bimetallic catalyst. Metal migration and surface segregation 39, as well as dissolution and dealloying 48-50 can induce alterations in the surface composition and consequently in the activity over time.

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Particularly dissolution is an important factor that needs to be considered in studies of solid-liquid interfaces, although typical rates of noble-metal dissolution are relatively low. However, they can be relevant over long periods (years) that these catalysts are in operation and as a consequence in economic considerations 51. Moreover, one needs to be aware of the extent of dissolution during short-term kinetic studies, as even minor surface changes can have severe impact.

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The development of a unique experimental technique that combines an online inductively coupled plasma mass spectrometry (ICP-MS) with an analytics electrochemical scanning flow cell (SFC) 52, 53 has permitted the time-resolved quantification of low amounts of dissolved elements during catalytic studies, and thus significantly contributed to fundamental understanding of dissolution/degradation processes, like the dissolution of bulk (foil or disk) noble metals 54, including different studies on gold 7, 55 and palladium 56. While polycrystalline bulk materials constitute model systems for understanding the fundamental dissolution mechanisms, the stability of real-application high surface area catalysts have not been comprehensively 3

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investigated 57. Furthermore, due to the complexity of alloys interface, the number of studies in bimetallic nanoparticle dissolution is limited 48, 58, 59.

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In this work, we report a time-resolved dissolution study on gold-palladium bimetallic nanoparticles supported directly onto an electrode. These were originally designed and synthesized for the hydrogen peroxide production for which, as mentioned above, the ensemble effect is of crucial importance. In particular, the dissolution onset potential, the effect of the upper potential limit, the scan rate and the influence of the reaction environment (different acidic media and gases) will be presented and discussed.

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2 Experimental

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2.1 Nanoparticles synthesis and characterization

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Au-Pd colloidal materials (Au, AuPd and Pd) were prepared following ref. 35. Initially, aqueous solution of desired concentration of NaBH4 (0.1 M), poly(vinyl alcohol) (PVA) (1 wt% aqueous solution, Aldrich, MW=10 000, 80% hydrolyzed), HAuCl4 . 3H2O and PdCl2 (both Johnson Matthey) were prepared. The PVA solution (1 wt %) was added with the following required amount (PVA/(Au + Pd) (w/w)=1.2). Finally, the freshly NaBH4 solution was added (0.1 M, NaBH4/(Au + Pd) (mol/mol)=5) yielding after 30 min generation, a dark-brown sol. The obtained solutions were concentrated using a rotory evaporator obtaining the Au, Pd and AuPd solutions with a concentration of 0.1 mg ml-1.

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The molar ratio of the AuPd sample was evaluated with ICPMS before and after electrochemical measurements. The as prepared or degraded catalysts were first dissolved in aqua regia (HNO3:HCl (1:3)) and then the obtained solutions were diluted in UPW in order to be measured with ICP-MS.

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Samples of the three-different catalyst were prepared for examination by transmission electron microscopy (TEM) by dispersing the as prepared sol gel solutions onto a lacey carbon film supported by a gold TEM grid (PLANO). To determine their particle sizes and composition, the prepared samples were then subjected to bright field contrast imaging and X-ray energy-dispersive spectrometry (XEDS) experiments using a JEOL 2200FS TEM operating at 200 kV.

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The electrode used for scanning flow cell (SFC) consisted of circular spots of catalyst that were printed onto glassy carbon plates starting from colloidal ink using a drop-ondemand printer (Nano-PlotterTM 2.0, GeSim). Each layer consisted of 200 drops. The single drops (volume 150–200 pl) were dropped in rapid succession using a piezoelectric pipette. Only one layer was used for all experiments, apart from the estimation of the dissolution onset potential where 4 layers were used.

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2.2 Electrochemical characterization

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All electrochemical dissolution experiments were performed in SFC similar to the one used in our previous work 52. The electrolytes were gas (Ar or O2) purged 0.1 M H2SO4 and 0.1 M HClO4, prepared by dilution of concentrated acid (Suprapur®, Merck) in ultrapure water (PureLab Plus system, Elga, 18 MΩ). The prepared catalyst films, a graphite rod and a Ag/AgCl were used as working (WE), counter (CE) and reference electrode (RE), respectively. Experimental parameters were controlled with a homemade LabVIEW software. The flow rate of the electrolyte was 193 μl min−1. The quantitative analysis of the electrolyte for the dissolved 197Au and for a 106Pd was performed with an ICP-MS (NexION 300X, Perkin Elmer), using as internal standards 187Re and 103Rh respectively.

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The influence of degradation in the oxide reduction peaks was studied with a rotating disk electrode (RDE) method in the two considered electrolytes. A home-built three electrode separate compartment electrochemical cell made of Teflon® was employed. The working electrode consisted in 2 ug of catalyst deposited onto a polished homemade Teflon tip with a glassy carbon disk (5 mm from MaTeck). A graphite rod and a Ag/AgCl were used as CE and RE respectively.

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All experiments were carried out at room temperature (approximately 24°C) and all the potentials reported in this work are referred to the reversible hydrogen electrode (RHE), which was measured for every experiment.

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3 Results and discussion

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A sol-immobilization method commonly employed in recent works 35 was used to prepare colloidal solutions of unsupported Au, Pd and AuPd (1:1 molar ration) catalysts. The latter consists of bimetallic random homogeneous gold palladium alloy with a facecentered cubic (fcc) structure as described in a previous publication 35.

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Representative bright field STEM micrographs (AuPd in Figure 1a) of nanoparticles deposited onto a lacey carbon gold coated TEM grid were acquired, in order to measure the average particle size and the size distribution (Figure 1b and summary in Table 1). The AuPd alloy composition was furthermore qualitatively confirmed by energy dispersive X-ray spectra (Figure 1a) and by ICP-MS analysis of the as synthesized AuPd catalyst.

171 172 Table 1 Particle size and specific surface area of the prepared materials investigated in this study median / nm mean / nm st. dev. ECSA* / m2 g-1 At (1l)** / mm2

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Au 4.2 4.6 ±2.1 AuPd 4.1 4.1 ±0.8 Pd 3.1 3.2 ±1.1 *ECSA refers to the catalyst specific surface area, which was calculated from the information S2); **At refers to the total surface area of per deposited layer (≈2 ng)

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Figure 1 (a) TEM micrographs of AuPd nanoparticles with relative EDX spectra (red for Au peak and green for Pd peak). (b) Corresponding AuPd particle size distribution. (c) Cyclic voltammogramms [0.1-1.6] VRHE of colloidal catalyst (1 printed layer) in Ar purged 0.1M HClO4. Scan rate: 200 mV s-1.

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The prepared catalyst colloidal ink is printed on a glassy carbon plate, resulting in an array of samples that can be measured using a scanning flow cell (SFC) with an opening of around 1 mm in diameter. The loading of a single printed layer is estimated from the droplet size and is approximately 2 ng. From the statistical average particle size and the loading the total surface area per deposited layer is calculated (At in Table1; see calculation in S1-S2 of SI).

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Prior to dissolution studies, argon background CVs are measured up to 1.6 VRHE with 200 mV s-1 of scan rate in 0.1 M HClO4 (perchloric acid) to characterize the freshly prepared samples (Figure 1c). High potentials (1.6 VRHE) are used here and in other measurements of this work due to the Au oxide reduction peak that is only accessible at high upper limit potentials (ULP > 1.5 VRHE) 55. The peak potential of the Pd oxide reduction is shifting to higher potentials when Pd is alloyed with Au, in accordance with previous works on AuPd alloys 17, 60-62. A shift to lower potential is also observed for the Au oxide reduction and this depends on the composition 17; however, according to literature this shift is less pronounced compared to the Pd oxide reduction 62. Analyzing these features, it might be possible to derive the real electroactive surface area of Au and Pd separately, i.e. by using (I) the charge under the oxide reduction peak 62, (II) the charge under the oxide formation peak or (III) the charge of the hydrogen under potential deposition (HUPD) 63. However, the estimation of the reduction peak for palladium, where different oxidized states are formed 61, and in general for alloys is challenging 60, 62. Furthermore, the presence of gold is reported to hinder the hydrogen bulk absorption and surface adsorption/desorption features 17 as well as the Pd-oxide reduction 60. Finally, in both 6

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cases, the integration of the charge could be affected by systematic errors related to the definition of the baseline. Therefore, we consider for these catalysts only the total surface area At calculated from the statistical particle size and loading (Table 1) as done also for Au-Pd catalysts in ref. 17. For AuPd this value is in the same order of magnitude (within the error) with the initial surface area estimated from the double layer capacity (see Figure S2 in SI).

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3.1 Dissolution Onset Potential

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Figure 2 Dissolution profiles of 4 printed layers of palladium (a) and gold (b) during a cyclic voltammogram between 0.05 and 1.5 VRHE in Ar purged 0.1 M HClO4 with a scan rate of 2 mV s-1. The dotted and full lines represent the pure metal and the alloyed metal, respectively. The corresponding integrated dissolution of the pure and alloyed Pd and Au is shown in the respective inset. Flow rate is 193 μL min−1.

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The transient dissolution of Au, Pd and AuPd alloy is studied by utilizing the SFC-ICP-MS (Figure 2). For this measurement 4 printed catalyst layers are used to better identify the dissolution onset potentials, since the deviation from the background signal is easier to be observed when more catalyst is used. Note that the number of layers can influence the specific dissolution 64, whereas it does not influence the onset potential. The Au and Pd dissolution during dynamic potential operation are initiated by the formation of the respective surface oxides as confirmed from the first CV of a freshly printed catalyst (Figure 1). Only the first cycle is reported here, and for this measurement the dissolution is normalized to the surface area (At in Table 1); note that for the following Figures 3 and 4 the dissolution is not normalized, since during a degradation measurement the nanoparticle surface area and its composition are steadily changing.

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For a better comparison with the pure metal counterparts in Figure 2a-b Au and Pd dissolutions are shown separately: the full and the dotted lines represent the alloy and the pure metal, respectively. The measured dissolution onset potentials are defined as the deviation from the background signal in the positive scan (see section S3 in SI). These are respectively ≈0.78 VRHE for pure Pd and ≈1.3 VRHE for pure Au. The value for pure Pd is in accordance with measurements on polycrystalline bulk metal, while a previous study on polycrystalline Au in perchloric acid showed values slightly higher than our Au nanoparticles 54. In Figure 2b the gold profile presents the typical two peaks corresponding to dissolution during anodic and cathodic scan. Several mechanisms of gold oxide formation and dissolution have been already thoroughly described, although the exact reaction pathway is still not clarified 7, 55.

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As already observed for polycrystalline material, the Pd dissolution per cycle (≈117 ng cm-2) is more extensive than gold and other noble metals 56, 65. Moreover, instead of two separate peaks resulting from oxidation and reduction the presence of a third and sometimes fourth peaks indicates that additional processes, recently clarified 56, play an important role. These can be related to the complex structure of Pd oxides, the oxidation state of Pd and the oxide’s chemical composition. Contradicting reports suggest, on the basis of CV measurements, that the formation of the first monolayer of Pd(II)-oxide occurs either in the 1.45–1.50 VRHE potential range or between 1.1–1.3 VRHE 61. At higher anodic potentials (>1.4 VRHE) a further oxidation occurs on the surface leading to Pd(IV)oxide 61, 66. Our experimental results confirm a change in the oxide composition due to the reduction of surface Pds(IV)-oxide to Pd(II)-oxide and Pd-metal followed by transient dissolution (first cathodic peak in Figure 2), which occurs at 1.1-1.2 VRHE. The second cathodic peak corresponds to the reduction of the remaining bulk Pdb(IV) oxide layer in a non-reversible process which occurs in parallel to Pd(II)-oxide reduction to Pd-metal and dissolution of Pd-metal 56.

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Concerning the behavior of the alloyed metals, controversial results regarding increased stability of metals in alloyed nanoparticles compared to the pure metals are reported 6771. In fact, some theoretical DFT calculations claim that the presence of an alloying element would induce a shift in the oxidation and dissolution potential, thus leading to a stabilization of the alloy. This is possibly related to a delayed coverage of O* and OH* intermediates. Often, the doping of Au is reported to have a positive effect in the stabilization of other noble metals such as Pt 72. Recently, however, Cherevko et al. showed that a Pt sub-monolayer on bulk Au is not stable, but rather shows significant dissolution of both Au and Pt similar to the pure polycrystalline elements 73. In our case, the Au and Pd dissolved masses in the alloy normalized by the nanoparticle total surface area (insets in Figure 2a-b) are in absolute terms approximately half of those for the pure metals (for Pd is ≈60%, for Au ≈50%). Considering that the nominal stoichiometry is 50% Au and 50% Pd, and assuming that the initial surface composition does not differ significantly, this suggests that the dissolution normalized by the respective nanoparticles surface area in the alloyed nanoparticles is approximately in line with the pure metals. Nevertheless, a possible non-homogeneity of the alloys and the difficulty in 8

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estimating the real surface composition make the interpretation of the results rather challenging. A study on a model surface would be therefore recommended to confirm/exclude the effect of gold on the overall dissolution per cycle.

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Interestingly, however, the dissolution onset potential of Pd in the alloyed catalyst (see Figure S3.2 in SI) is slightly higher (approximately 30 mV higher around 0.81 VRHE) compared to pure Pd. Similarly, Cherevko et al. showed that the onsets of Pt and Au dissolution after intermixing shift to slightly higher potential than the pure elements 73. Furthermore, in line with oxide reduction peak shift (Figure 1), the cathodic Pd dissolution of the alloyed material ends significantly earlier, one more time confirming a correlation between Pd-oxide reduction and cathodic dissolution processes. Therefore, alloying influences clearly the dissolution onset and final potentials, whereas no significant effect in the quantitative dissolution is observed.

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Changing the upper potential limit (UPL) or the scan rate has no influence on the dissolution onset potentials. Nevertheless, as shown for polycrystalline metals, the rate of dissolution and the shape of the dissolution peaks and profiles for the AuPd alloy are strictly related to the UPL (see Figure S4 in SI) and scan rate. At higher scan rates, it is for example not possible to distinguish the two cathodic peaks even at higher UPL as visible already from the cycles at 10 mV s-1 in Figure 3. The overlap between dissolution peaks at higher scan rates was previously observed for Pt polycrystalline and related to the technical limitations of the setup 74.

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In conclusion, considering the dissolution onset potentials, it is possible to define a stability window for bimetallic nanoparticles like AuPd catalyst: below the Pd onset potential (≈0.8 VRHE) virtually no metal is being leached out from the catalyst surface, so that the composition remains unchanged. Above 0.8 VRHE severe dissolution of Pd and Au occurs, which leads to changes in surface composition and long-term degradation of the catalyst. These considerations of course are not taking in account surface restructuring and metal migration, which might occur at low potentials.

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3.2 Influence of the acidic medium in dealloying

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Figure 3 Dissolution profiles of AuPd nanoparticles (1 layer) in Ar purged (a) 0.1M HClO4 and (b) 0.1M H2SO4 during 50 cyclic voltammograms between 0.1 and 1.6 VRHE with a scan rate of 200 mV s-1; some CVs at slower scan rate (10 mV s-1) were recorded to plot the dissolution cycle profiles with time (insets in a-b). The corresponding RDE cyclic voltammograms in Ar purged 0.1M HClO4 and 0.1M H2SO4 are shown in (c) and (d) respectively. The relative palladium and gold oxide reduction charges are displayed in the insets.

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Online dissolution and voltammetric profiles [0.1-1.6 VRHE] are recorded in Ar purged 0.1 M HClO4 and H2SO4 (Figure 3) to characterize the changes in dissolution rate and surface composition of the particles over time. Such a high overpotential is chosen in order to (I) accelerate the degradation and to (II) follow the gold reduction peak that is visible only with scans to high potentials. In Figure 3 the dissolution is not normalized to the surface area, since the total area and the surface composition is changing during the measurement due to dissolution and dealloying. For sake of comparison, the same measurement has been performed also with the pure metal counterparts (see Figure S5.3 in SI). The charges associated with the two characteristic reduction peaks in the profile are proportional to Au and Pd surface areas, respectively. However, as previously discussed, the extrapolation of surface area in alloys is ambiguous, therefore we simply report the associated reduction charges (Figure 3 cinset-dinset), which in any case are intended to be proportional to the Pd and Au surface areas.

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50 cycle profiles (1, 5, 10, 20, 30 and 50) with scan rate 200 mV s-1 are shown in Figure 3 c-d. The surface and its composition are changing rapidly as confirmed by the reduction charges: the Pd-oxide reduction peak decreases, while the Au-oxide increases in magnitude during the first cycles. At the same time, the amount of dissolved Pd is constantly dropping as a consequence of the decrease in surface Pd. As described in earlier reports 39, 60, 62, 75, 76, the voltammograms of the Au-Pd alloys change significantly during continuous potential cycling in acidic media to sufficiently high potentials. In the literature this was attributed to (I) Au migration to the surface 39, to (II) potential 10

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dependent Pd surface segregation 18 or to (III) selective Pd removal 62. Our results show that the main reason for surface Au enrichment is dealloying. Of course the other two, especially surface diffusion of gold atoms 77, cannot be completely excluded, however their role in this process is considered minor compared to dissolution. Indeed, Pd is dissolving at a much higher rate than Au during the first cycles; thus, Au is being increasingly exposed to the surface, hence forming a gold “skin” (Figure 4). While this might be positive for materials like PtM used for ORR leading to an increased Pt surface, in applications where the surface metal composition is crucial for activity and selectivity, dealloying needs to be avoided to retain the desired initial properties. Detailed information about the application are therefore required and these need to be compared to the bimetallic stability window. Nevertheless, dealloying can cause the formation of porous bimetallic structure (as shown from dealloyed PtNi nanocatalyst 78) that can lead to new interesting perspective as shown for gold nanoporous catalyst 79-81.

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Figure 4 Schematic representation of selective palladium dealloying, yielding a gold enriched surface composition. (a) Fresh prepared catalyst, (b) Pd dissolution and (c) Au-enriched surface after potential cycling.

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Interestingly, the palladium dissolution throughout the measurement is not simply decreasing quantitatively, but also the profile is changing as shown in the comparison of the cycles with slower scan rates (inset Figure 3 a-b). Indeed, the anodic dissolution onset potential is shifting positively from 0.9 VRHE of the first cycle to approximately 1.0 VRHE. Similar positive shift was observed for sub-monolayer of [email protected] dissolution 73. During the cathodic scan, the dissolution maxima (only one peak is distinguishable at this scan rate) as well as the dissolution final potential are slightly shifting to higher potentials. This is correlated to the decrease in Pd content with dissolution, which produces a more “intimate” mixed alloy with finely dispersed palladium in the gold matrix. Indeed, the Pd-O reduction peak potential in AuPd alloys is strictly correlated to the Pd content (Figure 1): the less palladium is present in the alloy, the higher the potential for Pd-O reduction is 17, which explains the change in dissolution maxima potentials.

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The catalyst dissolution behavior is here studied also in a different acidic media (sulfuric acid). The dissolution onset potential for alloyed Au (see Figure S5.2 in SI) is shifted of approximately 50 mV (≈1.25 VRHE in sulfuric acid and ≈1.3 VRHE in perchloric acid). This change is in accordance with measurements on polycrystalline gold electrodes 7, where 11

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a shift of about 100 mV was observed (≈1.3 VRHE in sulfuric acid and ≈1.4 VRHE in perchloric acid). On the other hand, the alloyed Pd behaves more like Pt: its dissolution onset potential is not changing, in accordance with other studies on polycrystalline Pt 7. Instead, the amount of dissolved palladium per cycle is changing significantly with the acid. Indeed, during the first cycle Pd is dissolved more in sulfuric acid (≈0.24 ng) compared to perchloric acid (≈0.13 ng). This difference in Pd removal is mirrored in the recorded CVs (Figure 3 c-d) by a faster decrease of the Pd-oxide reduction peak 39: in H2SO4 Pd disappears after 10 CVs whereas in HClO4 it is still detectable after 50 CVs. Therefore the enhanced electrodissolution of Pd depends on the nature of anions present in the electrolyte, as they facilitate the formation of products and/or intermediates 56, 61. In particular, based on the results reported in the literature, sulfuric acid promotes Pd electrodissolution more than perchloric acid 56.

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In both cases the last CVs (Figure 3 c-d after 50 cycles) indicate the presence of a gold enriched catalyst with probably a core-shell configuration, even though some isolated Pd atoms might still be present on the catalyst surface. Indeed, ICP-MS measurements of the degraded catalyst show a final Pd/Au ratio of 30/70 mol% after 50CVs to 1.6 VRHE in perchloric acid, thus confirming the presence of Pd in the core even after the degradation measurement.

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3.3 Influence of gas

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Figure 5 Pd dissolution profile of AuPd nanoparticles (1 printed layer) in O2 (full line) and Ar (dotted line) purged 0.1M HClO4. 10 cyclic voltammograms between 0.1 and 0.6 VRHE (below the dissolution potential) with a scan rate of 50 mV s-1 followed by open circuit potential (OCP).

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Online dissolution of AuPd nanoparticles is recorded in O2 purged 0.1M HClO4 (Figure 5). While no significant differences with Ar purged electrolyte are observed during potential cycling below the dissolution onset, the presence of oxygen leads to a shift in the open circuit potential (OCP). Namely, the OCP in oxygen purged electrolyte reaches approximately 0.9 VRHE, slightly above the measured dissolution onset potential, while in 12

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argon purged electrolyte it remains below 0.8 VRHE. Therefore, Pd is being dissolved at OCP in the presence of oxygen. Gas induced changes in alloys surface composition are already reported in heterogeneous catalysis literature 39, 82, which is commonly attributed to metal migration. According to our results, however, we suggest that also selective dissolution in the presence of different gases plays an important role in determining the surface composition. This is particularly relevant for the long term stability of bimetallic nanoparticles in reactions that requires gases such as O2, CO2, O3 that can cause high OCP values, or in reactors that are shut down frequently and air is able to diffuse in. Therefore, gas induced dealloying has also to be taken in consideration in heterogeneous catalysis, where no potential control is applied.

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4 Conclusions

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In summary, in this work we showed an extensive dissolution study on alloyed AuPd nanoparticles supported directly on the electrode. Using a unique technique that combines electrochemical measurements with online mass spectroscopy, we showed how the reaction environment is strongly influencing metal dissolution and dealloying. In particular, different electrolytes cause a significant variation in the dissolution rate depending on the nature of anions and/or cations present in the solution, and dissolved oxygen gas plays a key role in enhancing the dissolution rates by shifting the open circuit potential.

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Even though the interpretation of the results is challenging due to the difficulty in estimation of the precise surface composition in the alloyed catalyst, the quantitative normalized dissolution indicates that the dissolution of Au and Pd in the alloy is approximately half of the normalized dissolution of the metal counterparts. Considering the synthesized 1:1 molar ratio, no major stabilization of Pd was therefore observed. On the other hand, the measured Pd dissolution profiles are different for alloyed Pd compared to the pure metal: the anodic onset and cathodic final dissolution potential are shifted.

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A well-defined stability window can be deduced from the presented results below ≈0.8 VRHE and in the presence of a gas that cause low OCP, where no dissolution/dealloying of Pd occurs. In such cases changes in surface composition are only assignable to metal migration or segregation, which were not addressed in here. In contrast with other work in the literature we have shown that the main contribution to changes in surface composition is coming from selective dissolution rather than from metal migration above potentials of 0.8 VRHE. The faster dissolution rate of palladium compared to gold induces a gold surface enrichment. Indeed, the palladium dissolution decreases to almost zero and its profile is continuously changing throughout the degradation.

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AuPd catalyst were used here as a model system for dissolution and selective dealloying. However, the results and implications can be extended to any bimetallic system. With this study, it was shown that catalyst structure, surface composition, and thus activity, 13

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can change over time under reaction environments and conditions due to selective dissolution. In turn, dealloying could be also exploited positively with selective dissolution by subjecting particles to electrochemical conditions, in order to control and tune the catalyst surface composition. With this “activation” the bimetallic effects could be optimized to achieve and maintain enhanced catalytic activity.

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Our results imply that by defining the experimental conditions and thus influencing the dissolution, it might be possible to control precisely the surface composition. On the other hand, in real applications, where a determined surface composition is required to achieve an optimum in activity and selectivity, it is difficult to avoid dissolution and thus to control the bimetallic surface composition over the long reaction times. In fact, in fuel cells it is likely to have potential spikes which exceed the stability window during start and stop condition, while in heterogeneous catalysis mixtures of gases might lead to dealloying through changes in the potential of the system. In both cases this is detrimental for application based on reactions, where the coexistence of both metals on the surface is necessary (i.e. peroxide synthesis, alcohols oxidation, formic acid oxidation).

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In conclusion, having exhaustive dissolution/dealloying data combined with precise information about the reaction environment are of crucial importance to guarantee the performance and stability of all materials that rely on ensemble effects. Indeed, if potential fluctuations occur, the resulting dealloying can change in a short time dramatically their surface composition and therefore their activity.

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Acknowledgments

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E.P acknowledges financial support from the IMPRS-SurMat doctoral program. We thank The MAXNET Energy for the financial support. S.G. acknowledges financial support from BASF.

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References

454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469

1. 2. 3. 4. 5. 6. 7. 8. 9.

10.



S. J. Freakley, Q. He, C. J. Kiely and G. J. Hutchings, Catal Lett, 2015, 145, 71-79. M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem Lett, 1987, DOI: Doi 10.1246/Cl.1987.405, 405-408. A. S. K. Hashmi, Chem Rev, 2007, 107, 3180-3211. A. S. K. Hashmi, Angew Chem Int Edit, 2010, 49, 5232-5241. A. S. K. Hashmi and G. J. Hutchings, Angew Chem Int Edit, 2006, 45, 7896-7936. P. Rodriguez and M. T. M. Koper, Phys Chem Chem Phys, 2014, 16, 13583-13594. S. Cherevko, A. R. Zeradjanin, G. P. Keeley and K. J. J. Mayrhofer, J Electrochem Soc, 2014, 161, H822-H830. A. S. K. Hashmi, C. Lothschutz, R. Dopp, M. Ackermann, J. D. Becker, M. Rudolph, C. Scholz and F. Rominger, Adv Synth Catal, 2012, 354, 133-147. D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362-365. J. K. Edwards, B. Solsona, E. N. N, A. F. Carley, A. A. Herzing, C. J. Kiely and G. J. Hutchings, Science, 2009, 323, 1037-1041. 14

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Page 15 of 18

Catalysis Science & Technology View Article Online

470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518

11.

12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

L. Kesavan, R. Tiruvalam, M. H. Ab Rahim, M. I. bin Saiman, D. I. Enache, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, D. W. Knight, C. J. Kiely and G. J. Hutchings, Science, 2011, 331, 195-199. M. H. Ab Rahim, M. M. Forde, R. L. Jenkins, C. Hammond, Q. He, N. Dimitratos, J. A. Lopez-Sanchez, A. F. Carley, S. H. Taylor, D. J. Willock, D. M. Murphy, C. J. Kiely and G. J. Hutchings, Angew Chem Int Edit, 2013, 52, 1280-1284. S. T. Bliznakov, M. B. Vukmirovic, L. Yang, E. A. Sutter and R. R. Adzic, J Electrochem Soc, 2012, 159, F501-F506. J. L. Fernandez, V. Raghuveer, A. Manthiram and A. J. Bard, J Am Chem Soc, 2005, 127, 13100-13101. K. A. Kuttiyiel, K. Sasaki, D. Su, L. J. Wu, Y. M. Zhu and R. R. Adzic, Nat Commun, 2014, 5. Y. C. Xing, Y. Cai, M. B. Vukmirovic, W. P. Zhou, H. Karan, J. X. Wang and R. R. Adzic, J Phys Chem Lett, 2010, 1, 3238-3242. J. S. Jirkovsky, I. Panas, E. Ahlberg, M. Halasa, S. Romani and D. J. Schiffrin, J Am Chem Soc, 2011, 133, 19432-19441. J. S. Jirkovsky, I. Panas, S. Romani, E. Ahlberg and D. J. Schiffrin, J Phys Chem Lett, 2012, 3, 315-321. F. Yang, K. Cheng, T. H. Wu, Y. Zhang, J. L. Yin, G. L. Wang and D. X. Cao, J Power Sources, 2013, 233, 252-258. M. Nie, H. L. Tang, Z. D. Wei, S. P. Jiang and P. K. Shen, Electrochem Commun, 2007, 9, 2375-2379. Y. W. Lee, M. Kim, Y. Kim, S. W. Kang, J. H. Lee and S. W. Han, J Phys Chem C, 2010, 114, 7689-7693. C. H. Cui, J. W. Yu, H. H. Li, M. R. Gao, H. W. Liang and S. H. Yu, Acs Nano, 2011, 5, 4211-4218. X. Y. Lang, H. Guo, L. Y. Chen, A. Kudo, J. S. Yu, W. Zhang, A. Inoue and M. W. Chen, J Phys Chem C, 2010, 114, 2600-2603. A. S. K. Hashmi, R. Döpp, C. Lothschütz, M. Rudolph, D. Riedel and F. Rominger, Adv Synth Catal, 2010, 352, 1307-1314. A. S. K. Hashmi, M. Ghanbari, M. Rudolph and F. Rominger, Chemistry – A European Journal, 2012, 18, 8113-8119. A. S. K. Hashmi, C. Lothschütz, R. Döpp, M. Rudolph, T. D. Ramamurthi and F. Rominger, Angewandte Chemie International Edition, 2009, 48, 8243-8246. M. M. Hansmann, M. Pernpointner, R. Döpp and A. S. K. Hashmi, Chemistry – A European Journal, 2013, 19, 15290-15303. J. R. Kitchin, J. K. Norskov, M. A. Barteau and J. G. Chen, Phys Rev Lett, 2004, 93. J. A. Rodriguez and D. W. Goodman, Science, 1992, 257, 897-903. V. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. Ross, N. M. Markovic, J. Rossmeisl, J. Greeley and J. K. Norskov, Angew Chem Int Edit, 2006, 45, 2897-2901. M. Mavrikakis, B. Hammer and J. K. Norskov, Phys Rev Lett, 1998, 81, 2819-2822. L. A. Kibler, A. M. El-Aziz, R. Hoyer and D. M. Kolb, Angew Chem Int Edit, 2005, 44, 2080-2084. M. S. Chen, D. Kumar, C. W. Yi and D. W. Goodman, Science, 2005, 310, 291-293. F. Maroun, F. Ozanam, O. M. Magnussen and R. J. Behm, Science, 2001, 293, 18111814. J. Pritchard, L. Kesavan, M. Piccinini, Q. He, R. Tiruvalam, N. Dimitratos, J. A. Lopez-Sanchez, A. F. Carley, J. K. Edwards, C. J. Kiely and G. J. Hutchings, Langmuir, 2010, 26, 16568-16577.

15

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Page 16 of 18 View Article Online

519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554

36.

555 556 557 558 559 560 561 562 563 564 565 566 567

2011. 52. S. O. Klemm, A. A. Topalov, C. A. Laska and K. J. J. Mayrhofer, Electrochem Commun, 2011, 13, 1533-1535. 53. S. Mezzavilla, S. Cherevko, C. Baldizzone, E. Pizzutilo, G. Polymeros and K. J. J. Mayrhofer, ChemElectroChem, 2016, DOI: 10.1002/celc.201600170, n/a-n/a. 54. S. Cherevko, A. R. Zeradjanin, A. A. Topalov, N. Kulyk, I. Katsounaros and K. J. J. Mayrhofer, Chemcatchem, 2014, 6, 2219-2223. 55. S. Cherevko, A. A. Topalov, A. R. Zeradjanin, I. Katsounaros and K. J. J. Mayrhofer, Rsc Adv, 2013, 3, 16516-16527. 56. E. Pizzutilo, S. Geiger, S. J. Freakley, A. Mingers, S. Cherevko, G. J. Hutchings and K. J. J. Mayrhofer, Electrochim Acta, 2017, DOI: 10.1016/j.electacta.2017.01.127. 57. E. Pizzutilo, S. Geiger, J.-P. Grote, A. Mingers, K. J. J. Mayrhofer, M. Arenz and S. Cherevko, J Electrochem Soc, 2016, 163, F1510-F1514.

37.

38. 39. 40. 41. 42. 43. 44. 45. 46.

47.

48.

49. 50. 51.

S. Siahrostami, A. Verdaguer-Casadevall, M. Karamad, D. Deiana, P. Malacrida, B. Wickman, M. Escudero-Escribano, E. A. Paoli, R. Frydendal, T. W. Hansen, I. Chorkendorff, I. E. L. Stephens and J. Rossmeisl, Nat Mater, 2013, 12, 1137-1143. A. Verdaguer-Casadevall, D. Deiana, M. Karamad, S. Siahrostami, P. Malacrida, T. W. Hansen, J. Rossmeisl, I. Chorkendorff and I. E. L. Stephens, Nano Lett, 2014, 14, 1603-1608. P. Liu and J. K. Norskov, Phys Chem Chem Phys, 2001, 3, 3814-3818. C. N. Brodsky, A. P. Young, K. C. Ng, C. H. Kuo and C. K. Tsung, Acs Nano, 2014, 8, 9368-9378. S. Suzuki, T. Onodera, J. Kawaji, T. Mizukami, Y. Takamori, H. Daimon and M. Morishima, Ecs Transactions, 2010, 33, 321-332. T. Ghosh, B. M. Leonard, Q. Zhou and F. J. DiSalvo, Chem Mater, 2010, 22, 21902202. B. M. Leonard, Q. Zhou, D. N. Wu and F. J. DiSalvo, Chem Mater, 2011, 23, 11361146. V. R. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. Ross and N. M. Markovic, J Am Chem Soc, 2006, 128, 8813-8819. J. W. Hong, D. Kim, Y. W. Lee, M. Kim, S. W. Kang and S. W. Han, Angew Chem Int Edit, 2011, 50, 8876-8880. I. W. C. E. Arends and R. A. Sheldon, Appl Catal a-Gen, 2001, 212, 175-187. R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. I. Kimijima and N. Iwashita, Chem Rev, 2007, 107, 3904-3951. F. Tao, M. E. Grass, Y. W. Zhang, D. R. Butcher, F. Aksoy, S. Aloni, V. Altoe, S. Alayoglu, J. R. Renzas, C. K. Tsung, Z. W. Zhu, Z. Liu, M. Salmeron and G. A. Somorjai, J Am Chem Soc, 2010, 132, 8697-8703. S. Mezzavilla, C. Baldizzone, A.-C. Swertz, N. Hodnik, E. Pizzutilo, G. Polymeros, G. P. Keeley, J. Knossalla, M. Heggen, K. J. J. Mayrhofer and F. Schüth, Acs Catal, 2016, 6, 8058-8068. S. Koh and P. Strasser, J Am Chem Soc, 2007, 129, 12624-+. S. Rudi, L. Gan, C. H. Cui, M. Gliech and P. Strasser, J Electrochem Soc, 2015, 162, F403-F409. U. DoE, Hydrogen, Fuel Cells and Infrastructure Technologies Programm: Multiyear Research, Development and Demonstration Plan

16

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Catalysis Science & Technology View Article Online

568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612

58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72.

73. 74. 75. 76. 77. 78. 79.

80.

81. 82.

A. Pareek, G. N. Ankah, S. Cherevko, P. Ebbinghaus, K. J. J. Mayrhofer, A. Erbe and F. U. Renner, Rsc Adv, 2013, 3, 6586-6595. P. Jovanovic, A. Pavlisic, V. S. Selih, M. Sala, N. Hodnik, M. Bele, S. Hocevar and M. Gaberscek, Chemcatchem, 2014, 6, 449-453. H. Erikson, A. Sarapuu, J. Kozlova, L. Matisen, V. Sammelselg and K. Tammeveski, Electrocatalysis-Us, 2015, 6, 77-85. M. Grdeń, M. Łukaszewski, G. Jerkiewicz and A. Czerwiński, Electrochim Acta, 2008, 53, 7583-7598. M. Lukaszewski and A. Czerwinski, Electrochim Acta, 2003, 48, 2435-2445. S. Henning, J. Herranz and H. A. Gasteiger, J Electrochem Soc, 2015, 162, F178F189. G. P. Keeley, S. Cherevko and K. J. Mayrhofer, ChemElectroChem, 2015. D. A. J. Rand and R. Woods, J Electroanal Chem, 1972, 35, 209-&. A. N. Correia, L. H. Mascaro, S. A. S. Machado and L. A. Avaca, Electrochim Acta, 1997, 42, 493-495. V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A. Lucas, G. F. Wang, P. N. Ross and N. M. Markovic, Nat Mater, 2007, 6, 241-247. G. E. Ramirez-Caballero, Y. G. Ma, R. Callejas-Tovar and P. B. Balbuena, Phys Chem Chem Phys, 2010, 12, 2209-2218. K. Sasaki, H. Naohara, Y. Cai, Y. M. Choi, P. Liu, M. B. Vukmirovic, J. X. Wang and R. R. Adzic, Angew Chem Int Edit, 2010, 49, 8602-8607. K. Sasaki, H. Naohara, Y. M. Choi, Y. Cai, W. F. Chen, P. Liu and R. R. Adzic, Nat Commun, 2012, 3. J. Greeley and J. K. Norskov, Electrochim Acta, 2007, 52, 5829-5836. M. Gatalo, P. Jovanovič, G. Polymeros, J.-P. Grote, A. Pavlišič, F. Ruiz- Zepeda, V. S. Šelih, M. Šala, S. Hočevar, M. Bele, K. J. J. Mayrhofer, N. Hodnik and M. Gaberšček, Acs Catal, 2016, 6, 1630-1634. S. Cherevko, G. P. Keeley, N. Kulyk and K. J. J. Mayrhofer, J Electrochem Soc, 2016, 163, H228-H233. A. A. Topalov, S. Cherevko, A. R. Zeradjanin, J. C. Meier, I. Katsounaros and K. J. J. Mayrhofer, Chem Sci, 2014, 5, 631-638. D. A. J. Rand and R. Woods, J Electroanal Chem, 1972, 36, 57-&. R. Woods, Electrochim Acta, 1969, 14, 632-&. J. M. Dona and J. Gonzalezvelasco, J Phys Chem-Us, 1993, 97, 4714-4719. C. Baldizzone, L. Gan, N. Hodnik, G. P. Keeley, A. Kostka, M. Heggen, P. Strasser and K. J. J. Mayrhofer, Acs Catal, 2015, 5, 5000-5007. T. Fujita, P. Guan, K. McKenna, X. Lang, A. Hirata, L. Zhang, T. Tokunaga, S. Arai, Y. Yamamoto, N. Tanaka, Y. Ishikawa, N. Asao, Y. Yamamoto, J. Erlebacher and M. Chen, Nat Mater, 2012, 11, 775-780. N. Asao, Y. Ishikawa, N. Hatakeyama, Menggenbateer, Y. Yamamoto, M. Chen, W. Zhang and A. Inoue, Angewandte Chemie International Edition, 2010, 49, 1009310095. J. Biener, M. M. Biener, R. J. Madix and C. M. Friend, Acs Catal, 2015, 5, 6263-6270. S. Alayoglu, F. Tao, V. Altoe, C. Specht, Z. W. Zhu, F. Aksoy, D. R. Butcher, R. J. Renzas, Z. Liu and G. A. Somorjai, Catal Lett, 2011, 141, 633-640.

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Studying changes in surface composition of bimetallic (AuPd) catalyst under dealloying is of key importance to predict their stability during application.

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